Does anybody know why the 1930 Census is a partial census?

What do we do about our grandparents and greats who do not have a birth certificiate? In TN they were not kept when my grandparents were born?

I have not found out yet who my grandmother's father was. Her mother was not married to him and everytime her mother (my great grandmother) shows up on the census she is living with someone else, i.e., the White Dr she was the cook for, one time she was living with her brother in law. and the other time she was living with her parents.

My mother said that her mother's last name was Freeman and that her mother's last name was Trails, neither of them have shown up as Freeman or Trails, they have both showed up as Browns. My grandmother's (who I am searching to find out her father's name)name is Pearl (or Pearlis) Minnie Classis Lucy Iva Della Scruggs (married name).

Please help. Thanx, Joan Vanleer-Moore